Update description of presets, boot presets, FAQ on preserving segments with a boot preset

[softhack007]

Help wanted: The documentation part for presets seems outdated, and should be modernized for 0.14.x and beyond. For example, the preset documentation still explains what's "new in 0.11.x", and states that named presets should be created with direct json api calls. Today all this functionality is available from the UI.

The FAQ still mentions that preset 16 is special, which is not the case any more since at least wled 0.13.x.

https://github.com/wled/WLED-Docs/blob/5813b3a1349a8904ef36f37332fb6aafc85e2b27/docs/basics/faq.md?plain=1#L275-L278

https://github.com/wled/WLED-Docs/blob/main/docs/features/presets.md